Screen printing has been a popular way of decorating shirts, textiles, and other garments in recent years. Over the years, designs have evolved to become quite complicated requiring many layers of inks to be deposited on the textile. Drying or setting the ink after printing is commonly referred to as “flashing” within the textile printing industry. Plastisol ink is the most widely used ink system in t-shirt printing. Water-based inks are occasionally used but far less frequently. While water-based inks dry through evaporation, which is hastened by heat and air movement, plastisol has no solvents contained in its formulation and therefore requires only that the ink deposit is elevated to 230° Fahrenheit (110° Celsius) in order to “gel” the ink. A gelled ink deposit is dry to the touch but lacks any durability that a finished cured ink provides. Fully fused or cured plastisol is stretchable and resists cracking when stretched, whereas partially gelled/cured plastisol cracks easily if stretched. Fully curing plastisol is typically not attempted during the printing process but rather only after the garment has finished the printing process and is removed from the press. During curing, the garment is usually placed onto a conveyor that travels through a heated tunnel or oven designed to allow for the extra processing time to elevate the garment all the way to a 325 degree Fahrenheit (163° Celsius) temperature without scorching the material. Scorching of the textile or garment happens more easily then when short processing times are attempted at very high temperatures of 1000° Fahrenheit (538° Celsius) or more.
The plastisol ink is flashed or gelled during various stages of the screen printing process to avoid a number of undesirable issues. For example, when pressure is applied from a squeegee for applying additional colors to the garment, ink from prior stations that has not been flashed or gelled has a tendency to lift off and is deposited on the bottom of the screen. This lifting off of the ink in turn creates a number of issues. For instance, having the ink lift off causes the design or image to have less ink such that the resulting color is lighter. The ink can also smear, and the size of the lines and dots forming the design or image can expand.
It is therefore desirable to flash the ink as many times as possible at various stations of the screen printing machine to avoid these as well as other issues. However, due to the cost and size of screen printing presses as well as the market demand for more and more colorful artwork on darker or even black shirts, it is often times very difficult to flash or gel as many colors as a printer would like in order to control all of these potential issues. For example, the darker a background is, the greater the ink deposit needs to be in order to block the muting effects of the background and provide color brilliance. However, there is a greater risk for other issues when more ink is deposited.
Since current flash or cure units take up the space of a print station, the flash units eliminate the ability to print in the same station as is used for printing a color. Consequently, the printer loses color capability every time a flash unit is added to the print sequence. To add some perspective, during the early years oft-shirt printing (around the 1980's), presses were made with a total of 6 or 8 stations. When two stations are eliminated for loading and unloading the t-shirts, the presses of that era were only able to apply 4 to 6 colors. This made printing difficult for any dark colored garments. Since that time every few years the number of stations offered by machine manufacturers has grown to where machines are available with up to 20 plus stations. However, printers still try to print on garments with normally no more than 3 flash units and 2 flash units is the industry average. So few flash units are able to be used with acceptable results because the absorbency of the substrates on which the design or image is printed. For example, typical t-shirt material is absorbent such that it tends to hold the ink deposit in place when the pressure of another screen and squeegee is applied on top of it. Other less absorbent substrates like paper or even fabrics, such as waterproof nylon, tend to not hold an ink in place during the pressure of another print cycle but instead allow the ink to smear and pick up onto the bottom of the screen. With this less absorbent substrate, the deposit of ink becomes lighter and lighter, thereby losing color intensity, print detail etc.
These printing challenges can be further exacerbated by using additional specialty inks, such as glitter, puff, or heavy athletic look inks, that may require a heavier than normal deposit. Understanding these printing difficulties, the printer is often challenged to figure out where compromises can be made, such as in image clarity, color density, or texture malformations, when a screen printing machine is only equipped with a limited number of flash units. While buying a bigger screen printing machine would allow the printer to have more flash units, in most cases a bigger press is not a practical option. First of all, bigger screen printing machines are very expensive and are typically outside the budget of most printers. In addition, such large printing machines have a larger footprint and therefore occupy a greater floor space which might not be available in a particular printer's facility. Moreover, print artists tend to design the art for a garment based on how many print stations a screen printing machine has and rarely considers the need for some of the stations to be used instead for flashing or drying. Since many print shops cannot afford the cost of these large screen printing machines and lack the size of a facility that it requires to house these large machines, they often have to turn work away.
One way to address this issue is to use a procedure often referred to as “revolving” which is described in U.S. Pat. No. 5,595,113 to Daniel et al. A traditional approach uses a continuous production cycle in which one shirt is placed on the machine as one shirt is taken off, normally every 4 to 5 seconds. With the revolving technique, the printing process is changed into a batch production cycle that may take up to 10 minutes in order to produce 12 shirts. In this work around, the printer takes advantage of the few flash cure stations available and runs the garment around or under the available flash station(s) multiple times in order to dry each of the colors needed for a particular design. Often, the garments travel around 2 to 3, even as many as 10 times around printing machines before that batch of shirts is finished. This revolving printing technique is very time consuming and prematurely wears down the printing machine by requiring it to rotate or revolve 2 to 3 times (or even 10 times) as much as compared to the traditional continuous production cycle for producing the same amount of shirts or other garments. Thus, there is a need for improvement in this field.